Modern radio front-end architectures for third generation (3G) and fourth generation (4G) user equipment include a plurality of duplexers for each radio frequency (RF) band to be processed by a radio front-end architecture. As a proliferation of RF bands increase, a significant portion of a bill of materials (BOM) for a radio front-end also increases, which leads to additional financial costs and undesirable increases in circuit board area. FIG. 1 is a block diagram that shows a 3G radio front-end 10. As shown in the example of FIG. 1, it is not uncommon for a 3G front-end such as the 3G front-end 10 to need a first duplexer 12, a second duplexer 14 and a third duplexer 16 for three bands of operation. The first duplexer 12, the second duplexer 14, and the third duplexer 16 each feed a particular receive (RX) signal to a low noise amplifier (LNA) (not shown). Transmit (TX) power from a power amplifier (PA) (not shown) is selectively transferred through the first multiplexer 12, the second multiplexer 14, and the third duplexer 16 to an antenna 18 through a first single pole three throw (SP3T) switch 20 and a second SP3T switch 22. Even more troublesome than the BOM for 3G is the arrival of 4G in which up to eight duplexers may be needed.
At present, research efforts are underway to realize tunable duplexers using micro-electro-mechanical (MEMS) tunable resonators. One approach for realizing a tunable duplexer is based upon a hybrid transformer that is described in a related art paper entitled “A Tunable Integrated Duplexer with 50 dB Isolation in 40 nm CMOS” by M. Mikhemar, H. Darabi and A. Abidi from ISSCC2009.
FIG. 2 is a simplified schematic diagram of a related art hybrid transformer 24 described in the related art paper. The related art hybrid transformer 24 includes an autotransformer 26 having a transmit (TX) port 28 and a receive (RX) port 30 and an antenna port 32. The autotransformer 26 includes a first winding 34, a second winding 36, and a tap 38. This related art approach provides electrical isolation between the TX port 28 and the RX port 30 if the resistance value of a balanced resistor RBAL coupled between the TX port 28 and the RX port 30 is equal to four times a load resistance RL, where RL represents an antenna load resistor.
While the hybrid transformer 24 achieves its objective of tunability, it does so with a significant inefficiency by dissipating at last half of the energy passing through the TX port 28 and the RX port 30. The energy is dissipated by the RBAL resistor, which gives the hybrid transformer 24 an insertion loss of at least −3 dB with an assumption that the autotransformer 26 is ideal. Also troublesome is a need to dynamically tune RBAL relative to antenna voltage standing wave ratio (VSWR) changes which results dynamic changes in the value of RL. As such, a relatively expensive adaptive tuning circuit (not shown) is needed to tune RBAL to match the dynamic changes in the value of RL. What is needed is a hybrid transformer duplexer apparatus that does not create a −3 dB insertion loss by having balanced resistor RBAL.